tail queue是bdb中用的最多的数据结构. 定义在 src/dbinc/queue.h:
注: TRACEBUF,QMD_TRACE_HEAD等是为了 queue代码的debug, 这里移除出去.
数据结构
========
```
// type 即为 用户定义的 类型. 比如 db.h,
// struct __db_txn {
// * TAILQ_HEAD(__kids, __db_txn) kids;
// */
// struct __kids { type 即为 _db_txn. 问题: 用什么不直接用 TAILQ_HEAD(__kids, __db_txn) kids?
// struct __db_txn *tqh_first;
// struct __db_txn **tqh_last;
// } kids;
#define TAILQ_HEAD(name, type) \ // 理解为 "头结点"
struct name { \
struct type *tqh_first; /* first element */ \ // 此为具体的元素, 如 __db_txn
struct type **tqh_last; /* addr of last next element */ \ // "最后一个元素的tqe_next" 的地址
}
#define TAILQ_ENTRY(type) \
struct { \
struct type *tqe_next; /* next element */ \ // 此为具体的元素, 如 __db_txn
struct type **tqe_prev; /* address of previous next element */ \ "前一个元素的tqe_next" 的地址
}
```
方法定义
=========
```
#define TAILQ_EMPTY(head) ((head)->tqh_first == NULL)
#define TAILQ_FIRST(head) ((head)->tqh_first)
/**
* 这里做了一个trick. 看TAILQ_HEAD 和TAILQ_ENTRY的定义, 只有 TAILQ_HEAD给了struct name.
* 所以 强制转为 `(struct headname *)`, (而不是 `(struct entryname *)`). 但是 TAILQ_HEAD 和
* TAILQ_ENTRY 的定义是相同的, `(head)->tqh_last)` 为 最后一个元素的 tqe_next 的地址,
* 所以如果有 `struct entryname`的话, 即为 此 `struct entryname` 的地址.
* `(((struct headname *)((head)->tqh_last))->tqh_last)` 即为:
* `(((struct entryname *)((head)->tqh_last))->tqe_prev)`. 倒数第二个元素的 tqe_next.
* TAILQ_PREV 的定义也是这么理解.
*/
#define TAILQ_LAST(head, headname) \
(*(((struct headname *)((head)->tqh_last))->tqh_last))
#define TAILQ_NEXT(elm, field) ((elm)->field.tqe_next)
/**
* field 为type中, queue entry的field. 其 `(elm)->field.tqe_prev)` 为 前一个元素的 tqe_next 的地址,
* 所以如果有 `struct entryname`的话, 即为 前一个元素的 queue entry field 的地址.
* `(((struct headname *)((elm)->field.tqe_prev))->tqh_last)` 即为:
* `(((struct entryname *)((elm)->field.tqe_prev))->tqe_prev)`. 前第二个元素的 tqe_next.
*/
#define TAILQ_PREV(elm, headname, field) \
(*(((struct headname *)((elm)->field.tqe_prev))->tqh_last))
#define TAILQ_FOREACH(var, head, field) \
for ((var) = TAILQ_FIRST((head)); \
(var); \
(var) = TAILQ_NEXT((var), field))
#define TAILQ_INIT(head) do {
TAILQ_FIRST((head)) = NULL; \
(head)->tqh_last = &TAILQ_FIRST((head)); \ // tqh_last 为 头结点本身的 tqh_first指针的地址
} while ()
#define TAILQ_CONCAT(head1, head2, field) do { \ // 合并两个queue
if (!TAILQ_EMPTY(head2)) { \ // 若q2 不为空
*(head1)->tqh_last = (head2)->tqh_first;\ // 调整 q1 末尾节点的 tqe_next
(head2)->tqh_first->field.tqe_prev = (head1)->tqh_last; \ // 调整q2 第一个节点的tqe_prev
(head1)->tqh_last = (head2)->tqh_last; \
TAILQ_INIT((head2)); \ // 重置q2的头结点
} \
} while ()
#define TAILQ_INSERT_AFTER(head, listelm, elm, field) do { \ // 将elm 插到 listelm 后面
if ((TAILQ_NEXT((elm), field) = TAILQ_NEXT((listelm), field)) != NULL)\ // elm->next=listelm->next
TAILQ_NEXT((elm), field)->field.tqe_prev = \ // 若listelm不为队尾, 调整elm->next->pre
&TAILQ_NEXT((elm), field); \
else \
(head)->tqh_last = &TAILQ_NEXT((elm), field); \ // listelm为队尾, 调整tqh_last
TAILQ_NEXT((listelm), field) = (elm); \ // 调整listelm->next
(elm)->field.tqe_prev = &TAILQ_NEXT((listelm), field); \ // 调整elm->pre
} while ()
#define TAILQ_INSERT_BEFORE(listelm, elm, field) do { \
(elm)->field.tqe_prev = (listelm)->field.tqe_prev; \
TAILQ_NEXT((elm), field) = (listelm); \
*(listelm)->field.tqe_prev = (elm); \ // 若listelm为第一个元素, 则tqe_prev为 tqh_first \
(listelm)->field.tqe_prev = &TAILQ_NEXT((elm), field); \
} while ()
#define TAILQ_INSERT_HEAD(head, elm, field) do { \
if ((TAILQ_NEXT((elm), field) = TAILQ_FIRST((head))) != NULL) \
TAILQ_FIRST((head))->field.tqe_prev = \
&TAILQ_NEXT((elm), field); \
else \
(head)->tqh_last = &TAILQ_NEXT((elm), field); \ // queue为空的情况
TAILQ_FIRST((head)) = (elm); \
(elm)->field.tqe_prev = &TAILQ_FIRST((head)); \ // 第一个元素的 tqe_prev 指向头结点的tqh_first
} while ()
#define TAILQ_INSERT_TAIL(head, elm, field) do { \
TAILQ_NEXT((elm), field) = NULL; \
(elm)->field.tqe_prev = (head)->tqh_last; \ // 若queue为空, tqh_last 指向 tqh_first
*(head)->tqh_last = (elm); \
(head)->tqh_last = &TAILQ_NEXT((elm), field); \
} while ()
#define TAILQ_REMOVE(head, elm, field) do { \
if ((TAILQ_NEXT((elm), field)) != NULL) \ // elm 不为队尾, 调整elm->next->pre
TAILQ_NEXT((elm), field)->field.tqe_prev = \
(elm)->field.tqe_prev; \
else \
(head)->tqh_last = (elm)->field.tqe_prev; \ // elm 为队尾, 调整tqh_last
*(elm)->field.tqe_prev = TAILQ_NEXT((elm), field); \ // 若 elm为队首, 其tqe_prev为 tqh_first
} while ()
```